The present invention relates to the protection of electronic circuits from damage by electrostatic discharges when a circuit card or module is plugged into a larger assembly.
Electrostatic discharges cause increasing fatalities among electronic circuits, especially as the line spacing of integrated circuits becomes smaller and smaller, and as user-installed plug-in circuit cards become more common in equipment such as personal and even midrange computers. Conductive plastic bags protect the cards during transportation, but the potential difference between a card and the assembly it plugs into might be thousands of volts at the instant of insertion. For a long time, the standard approach to solving this problem has been to provide the card socket with one pin, or the card with a conductive guide or stiffener, which contacts the conductive assembly frame first, and thus discharges the triboelectric charge on the card frame or on the user's hand as it contacts the frame. An improved approach connects the card member directly to a ground line in the circuit residing on the card, so that the circuit itself is tied to the potential of the frame. These solutions, however, still allow high-voltage discharges or sparks from the card to the assembly at some point. Other approaches, such as shorting fingers which move away from socket contacts when a card is inserted, or intricate networks of very small gaps, or resistors and diodes in signal leads, are expensive to implement and not always completely effective.